Recent donors to the Conservative party are pledging large sums to the UK Independence party and have contributed funds to help fight 1,500 seats at next month's local elections, according to the party's treasurer.

In a rare interview, Stuart Wheeler, the gambling tycoon in charge of Ukip's funds, said former Tory backers had promised big donations for next year's European elections. Some had already made smaller donations that would contribute towards the £200,000 to be spent on advertising in local elections over the next few weeks, he said.

The anti-EU party led by Nigel Farage aims to build on its performance in February's Eastleigh byelection, where it pushed the Tories into third place. Senior Tories expect heavy losses as 2,362 seats will be contested in 34 county councils and new unitary authorities.

Wheeler, who himself gave more than £5.5m to the Conservatives before being expelled from the party three years ago, said Ukip was taking the local elections seriously while aiming for a bigger push in the 2014 European elections.

"Some of [our new donors] were Conservatives but many are unwilling to go public. Any donation of more than £7,500 has to go on the electoral commission website. Quite a few give us exactly £7,500 because they don't want [their identities] to be known," he said.

"There is one potential donor who said he will give us £1m provided one or two other people do as well for the 2014 elections. The person concerned has been a big Conservative donor. People don't always do what they say they are going to do but we think he will."

Wheeler, 78, was expelled by the Tories by email for donating £100,000 to Ukip in 2009. He still bristles at the fact that Eric Pickles, then Tory party chairman, claimed in an interview that he was sacked by telephone. "It is difficult to think about anything less important than how I was expelled, so why did he make a thing about it?" he said.

The Tories have not given up hope on winning Wheeler back, recognising that he is extremely well connected and that his £5m donation was the largest single pledge ever given to a political party.

Six weeks ago, Wheeler said, a senior Conservative whom he had tried to recruit for Ukip tried to persuade him to rejoin the Tories. "It was quite a reversal," he said. He has held meetings with seven MPs who he says are considering leaving the Conservatives.

Wheeler said about 30 councillors had switched from other parties to Ukip this year, around 20 of whom were Tories. Asked whether he believed that a Tory MP would defect to Ukip before the next general election, he replied: "I think it is slightly more likely than not, but one has to remember that an MP who defects is more likely to lose his seat at the next election."

Under Farage, the party is attempting to transform itself into a more potent electoral force by recruiting a new election adviser and expanding into new offices. As of a few weeks ago, the party's accounts showed a healthy balance of £340,000, Wheeler said.

"We have a membership now of around 25,000, up 5,000 from the start of the year, and we are asking for £30 from each member. We don't want to underestimate that as a source of income."

The Guardian disclosed last month that Nikki Sinclaire, a former Ukip MEP for the West Midlands, had accused Farage of putting her under pressure to sign up for UKIP's European grouping in 2010 so that the party could access extra funds meant for a new political grouping without her support. She has now asked the EU counter-fraud unit, Olaf, to investigate her claims and is due to meet officials.

Some backbench Tory MPs claim that a poor showing next month could trigger unrest against David Cameron's leadership. Three-quarters of the seats involve Tory v Lib-Dem contests, including in Chelmsford and Colchester in Essex, Lewes and Eastbourne in East Sussex, the Vale of the White Horse in Oxfordshire, and in parts of Warwickshire. Labour aims to take full control of Derbyshire, Nottinghamshire and Lancashire, and perhaps to win Staffordshire.